The ability to measure cardiac output, or blood flow, has long been recognized as being of substantial use. Blood flow is defined as the volume of blood flow over a unit of time, and if measured at the output of the heart, is a measure of cardiac output. Measurement of blood flow is useful in conjunction with cardiac pacemakers, as well as cardiac defibrillators and other cardioverters, and cardiac diagnostic instruments.
A primary prior art method of determining blood flow is that of thermodilution, where a bolus of blood at an elevated temperature is generated, such as by heating the blood with an electrical current between a pair of electrodes. A temperature sensor located in the blood stream downstream from electrodes produces an output in accordance with a known thermodilution curve, providing means for measuring the rate of blood flow between the heating the blood and the measurement point. From this, flow, or cardiac output, can be determined.
More recently, other techniques for determining blood flow by thermal sensing have been disclosed. For example, in U.S. Pat. No. 5,174,299, the disclosed technique is that of heating the flowing blood, and measuring the temperature of the blood at two points. Assuming that the flow is inversely proportional to the temperature difference between the two points, the temperature difference can be correlated into blood flow. See also, for example, U.S. Pat. No. 5,493,100, representative of heating blood by coupling a drive signal to a thermistor placed in the blood stream, and then sensing subsequent time variations. However, these techniques require the complexity of generating heat within the blood stream in order to establish the measurement conditions.
It is known that the temperature of the blood in the heart varies cyclically, i.e., with systole and diastole. Blood that has passed through the lungs is relatively cooled due to exchange with the air, and then when it comes back into the heart it is heated by the cardiac muscle. After ejection of the blood, the heart is again filled with relatively new cool blood, which then warms until the next heart contraction, producing cyclical variations. Such temperature variations are rather small, in the order of less than 1.degree. C., but can be detected by a fast time response sensor. These observations are the basis of U.S. Pat. No. 5,336,244, which discloses a pacemaker system having a fast response temperature sensor located in the heart for generating cycle-by-cycle variation of temperature indicative of heart contractions, enabling capture detection.